Irrespective of the combustion engine design (e.g. reciprocating engine, rotary piston engine or free-piston engine), noises are generated resulting from the consecutive working cycles (in particular intake and compression of a fuel/air mixture, power and exhaust of the combusted fuel/air mixture), on the one hand, these noises pass through the combustion engine as structure-borne sound and are then radiated as airborne sound from the outside of the combustion engine, on the other hand, these noises are passing as airborne sound together with the combusted fuel/air mixture through an exhaust system of the combustion engine.
These noises are frequently perceived as disadvantageous, on the one hand, legal provisions for noise abatement exist, which have to be complied with by the manufacturers of vehicles operated with combustion engines. These legal provisions normally specify a maximum permissible sound pressure during the operation of the vehicle. Manufacturers, on the other hand, try to make sure that the vehicles operated with combustion engines they produce have a characteristic noise emission, intended to match the image of the respective producer and to appeal to customers. With modern engines that have low volumetric displacement, this characteristic noise emission can frequently no longer be ensured by ordinary means.
The noises which are passing through the combustion engine as structure-borne sound can be attenuated easily and are there-fore no problem with respect to noise abatement, as a rule.
The noises passing through the exhaust system as airborne sound together with the combusted fuel/air mixture are reduced by mufflers positioned upstream of the rear opening of the exhaust system. These mufflers may be positioned down-stream of catalytic converters, if present. Such mufflers can operate according to the absorption principle and/or reflec-tion principle, for example. Both operating methods have the disadvantage that they require a comparatively large volume and create relatively high resistance against the combusted fuel/air mixture, which means that the overall efficiency of the vehicle drops, while the fuel consumption increases.
As an alternative or in addition to mufflers, so-called anti-sound systems are being developed for some time, which superimpose electroacoustically generated anti-sound on airborne sound generated in the combustion engine and passing through the exhaust system. Such systems are known, for example, from the documents U.S. Pat. Nos. 4,177,874, 5,229,556, 5,233,137, 5,343,533, 5,336,856, 5,432,857, 5,600,106, 5,619,020, EP 0 373 188, EP 0 674 097, EP 0 755 045, EP 0 916 817, EP 1 055 804, EP 1 627 996, DE 197 51 596, DE 10 2006 042 224, DE 10 2008 018 085 and DE 10 2009 031 848.
Such anti-sound systems normally utilize a so-called Filtered-x Least Mean Squares (FxLMS) algorithm, which endeavors to control an error signal down to zero. This error signal is measured by means of an error microphone. The error signal is endeavored to be controlled down to zero by the output of sound by means of at least one loudspeaker that is connected by a fluid connection with the exhaust system.
In order to accomplish a destructive interference of the sound waves of the airborne sound generated by the combustion engine and conducted in the exhaust system and the anti-sound generated from the loudspeaker, the sound waves originating from the loudspeaker must correspond to the sound waves generated by the combustion engine and conducted in the exhaust system in terms of amplitude and frequency. However, the sound waves originating from the loudspeaker must comprise a phase shift of 180° relative to the airborne sound generated by the combustion engine and conducted in the exhaust system. The anti-sound for each frequency band of the airborne sound conducted in the exhaust pipe is calculated separately by means of the FxLMS algorithm, by determining a suitable frequency and phase position of two sine wave oscillations that are shifted relative to one another by 90°, and by calculating the amplitudes for these sine wave oscillations. The purpose of anti-sound systems is that the sound cancellation is audible and measurable at least outside of the exhaust system, but also inside of it, if necessary. In this document, the term anti-sound is used to distinguish the sound generated by the loudspeaker from the airborne sound generated by the combustion engine and conducted in the exhaust system. When considered by itself, anti-sound involves normal airborne sound.
A respective anti-sound system is supplied by the company J. Eberspächer GmbH & Co. KG, Eberspächerstrasse 24, 73730 Esslingen, Germany.
It is a disadvantage with known anti-sound systems for exhaust systems that the continuous operation of the loudspeaker can produce a thermal overload of the loudspeaker and especially of an oscillator coil of the loudspeaker and/or a mechanical overload (of a diaphragm or spider, for example) of the loudspeaker.
To prevent a thermal overload of the oscillator coil of a loudspeaker, it is proposed in WO 02/21879 to calculate the expected heating up of the oscillator coil when a signal is provided to the loudspeaker by means of a mathematical model of the thermal behavior of the loudspeaker and in particular of the oscillator coil, and to reduce the amplitude of the signal provided to the loudspeaker if necessary such, that a specified temperature of the oscillator coil will not be exceeded.
The solution proposed from WO 02/21879, however, is not suitable for loudspeakers of anti-sound systems for exhaust systems. When the signal provided to the loudspeaker is reduced in its amplitude, it can no longer be ensured that the legal provisions with respect to the maximum permissible sound pressure for the operation of the vehicle can be complied with. Moreover WO 02/21879 does not consider any mechanical overload.